The fastest growing Internet language by the number of users is Arabic. With over 5,000% growth since 2000, Arabic is now the fourth language on the Internet with around 150 million users.
The top 3 Arab-speaking countries by the number of users are Egypt, Morocco and Saudi Arabia. Note that Algeria, Iraq and Sudan have a larger overall population than Saudi Arabia, but Internet penetration is still very low in these countries.
Source: Internet World Stats